k = 0.693/t1/2k = approx 0.09 but you should get a more accurate number.

Convert 140 mg to number of atoms initially (at time = 0)
Use ln(No/N) = kt to determine N = number of atoms after four hours.
The difference between initial and after 4 hours is atoms that decayed in 4 hours.
Then 3.7E10 decays per second = 1 Ci